The present disclosure relates to a light guide, an illuminating device, and an image reading device, and more particularly to a technique of reflecting light introduced into a light guide toward an output surface.
Some of existing image forming apparatuses, for example those incorporated in a multifunction peripheral, employ a linear light source composed of a rod-shaped light guide made of a resin and LEDs that emit light into the light guide through an end portion thereof in a longitudinal direction, as light source unit for an image reading device such as a scanner. The light source unit has to illuminate a document to be read in a linear shape, in synchronization with the reading operation of a line sensor. For such purpose, a light reflecting or scattering pattern is provided on a reflecting surface opposed to an output surface of the light guide from which the light is emitted outward, so as to deflect, with the pattern, the light introduced into the light guide toward the output surface, thereby directing the linear-shaped illuminating light toward the document to be read.
Here, in the case of the combination of the rod-shaped light guide and the LEDs, the illuminating intensity is different between direct light that directly reaches the light reflecting or scattering pattern surface from the LED thus to be deflected, and indirect light totally reflected once or more times by the outer periphery of the light guide and then deflected by the light reflecting or scattering pattern surface. In a region close to the incident surface in the main scanning direction, on which the light from the LED is incident, uniform illuminance is unable to be obtained in the illuminating light emitted from different positions of the light guide and the output surface in the longitudinal direction. Accordingly, an illuminating unit has been known that includes a light guide having a polygonal cross-section instead of circular and formed in a heteromorphic shape in the main scanning direction, so as to restrict the direct light from the LED from reaching the light reflecting or scattering pattern surface.
In addition, an illuminating unit has also been known that includes two prisms having different shapes provided on the reflecting surface, so as to make the illuminance distribution of the direct light and the indirect light uniform.